1. Field of the Invention
The present invention relates to a photodetector for measuring the intensity of light.
2. Related Background Art
In order to measure the intensity of light, widely used are photodiodes which generate electric charges in response to the quantity of light received thereby and output thus generated charges as a current signal. When such photodiodes are used for accurately measuring the intensity of light, their dark current is always a matter of concern. Hence, there have been proposed systems for reducing or cancelling the dark current according to kinds of photodiodes.
According to one of the proposed systems (hereinafter referred to as conventional method 1), the photodiode is cooled in view of the fact that the dark current is reduced when the operating temperature of the photodiode is lowered. Also, there is a system in which a light chopper is used, a light-shielded dummy photodiode is provided, or an object to be measured is irradiated with pulse light, so as to evaluate, as a dark current, the output current of the photodiode when there is no light incident thereon, and thus evaluated dark current is subtracted from an output current of the photodiode when there is light incident thereon, thereby yielding a current signal corresponding to the intensity of light to be measured (Japanese Patent Application Laid-Open Nos. 1-2322886, 55-160478, and the like; hereinafter referred to as conventional method 2).
Further, an apparatus (hereinafter referred to as conventional example) combining conventional methods 1 and 2 is disclosed in Japanese Patent Application Laid-Open No. 61-255060.
In the conventional photodetectors, since their dark current components are reduced by the above-mentioned systems, there may be shortcomings such as those noted in the following.
In conventional method 1, since it is necessary to provide a cooling section in order to cool the photodiode, the photodetector has a larger size. Further, when the dark current is to be reduced by cooling, a sufficient S/N ratio may not be secured in most cases that the light to be measured is weak.
In conventional method 2, though a favorable S/N ratio can be secured in cases that the current level generated as the light is received (hereinafter also referred to as signal current level) and the dark current level are on the same order, it may not be secured in cases that the signal current level is much lower than the dark current level, e.g., in the case of an InGaAs photodiode.
Accordingly, even the apparatus of the conventional example adopting both of conventional methods 1 and 2 has difficulty in performing accurate measurement when a dark current component occupies the major part of the current from the cooled photodiode.
In view of the foregoing, it is an object of the present invention to provide a photodetector which can secure a favorable S/N ratio even in cases that a dark current component occupies the major part of the output current of a photodiode.
It is another object of the present invention to provide a solid-state imaging apparatus which can capture an incident light image while securing a favorable S/N ratio even in cases where a dark current component occupies the major part of the output current of a photodiode corresponding to a pixel.